It is known that if a sensitized paper (for example a paper coated with zinc oxide) is subjected to an electrical charge the impregnation of the paper by the zinc oxide will allow the paper to retain the charge uniformly over the surface of the paper. However the charge is dissipated on exposing the paper to light and this fact is used in building up an electrostatic image of a document which has to be copied by exposing the charged sensitized paper to an image of the document so that the charge is dissipated in those regions of the image which are light and is not dissipated or only partially dissipated in those regions which are dark or partially dark. The electrostatic charge pattern is used to attract to the paper in those places where the charge remains, carbon black or minute granules of ink and the print is obtained by fixing the carbon black or ink onto the paper by partially impregnating the paper with the carbon black or ink typically by passing the paper bearing the printing medium between rollers.
Whilst some documents can be copied by projecting light through the document from the front to the back so as to form a correct image of the document on the surface of the sensitized paper, the majority of documents cannot be used in this way to produce a correct image of the printing or other information carried by the document usually because the document is either too thick or more commonly, there is printing or other information bearing indicia on both sides of the document and if the latter were illuminated images of the printing or other information contained on both sides of the document will be obtained superimposed one on the other on the surface of the sensitized paper and the result is useless.